The invention relates to alkaline manganese dioxide electrochemical cells, and, in particular, to cells having a positive current collector, or can, comprising on its inner surface a resin that comprises a silicon compound. Cells prepared in accordance with the invention exhibit improved performance characteristics in high rate applications by reducing the high resistance between the cathode and the positive current collector.
Small primary electrochemical cells have been commercially available for more than a century. Originally, all small commercially available primary electrochemical cells and batteries were of the zinc carbon type. However, the need for a higher capacity primary battery system led to the development of alkaline batteries. These batteries typically use an alkaline electrolyte instead of ammonium chloride and zinc chloride dissolved in water. Within the last two decades alkaline electrochemical cells have become a tremendous commercial success. In fact, sales of alkaline batteries now exceed those of zinc carbon batteries in the United States.
The most commercially successful alkaline batteries have been cylindrical cells of the well known "AAA," "AA," "C" and "D" sizes. Generally, such alkaline cylindrical batteries comprise a cathode which is a mixture of manganese dioxide, MnO.sub.2, and a carbonaceous material, typically graphite. In some cylindrical alkaline cells, this cathode mixture, which is often wetted with electrolyte, is compressed into annular rings. The cathode is placed into a metallic container which also serves as the positive current collector. Anodes of alkaline electrochemical cells usually comprise powdered zinc in some type of gel, usually carboxymethylcellulose. The anodic current collector, usually a brass pin, is placed in electrical contact with the anode. The anode and the cathode is of such alkaline cells are usually separated by a separator comprised of non-woven, inert fabric.
As with other electrochemical cell systems, a decrease in the internal cell resistance of alkaline electrochemical cells increases cell performance. It is generally agreed that much of the internal resistance in alkaline electrochemical cells results from contact resistance, i.e., poor electrical contact, between the cathode and the positive current collector and from the electrical resistance of the positive current collector. Consequently, it is desirous to provide for an alkaline electrochemical cell which has a positive current collector with low electrical resistance and good electrical connection between the positive current collector and the cathode.
One method of obtaining the desired electrical contact between the positive current collector and the cathode mix is to create high pressure at the interface between the two members. Hosfield, in U.S. Pat. No. 3,156,749, obtains high pressure contact through forming a cylindrical battery cathode by impact molding it within the current collector. High pressure contact can also be achieved by inserting annular rings of cathode mix into the positive current collector, which has an inside diameter less than the outside diameter of the cathode rings.
Even with good electrical connection realized through high pressure contact, it has been long recognized that contact resistance between the cathode and an untreated steel current collector causes a reduction in the performance of alkaline dry cell batteries. This resistance, which is known to increase during storage especially at high temperatures, is believed to be a function of the amount of oxide formed at the cathode-current collector interface. It is believed that the alkaline electrolyte reacts with the surface of the current collector to form a solid oxide.
A number of solutions to this increased resistance, e.g. oxide formation, have been suggested. For example, Ruben, in U.S. Pat. No. 3,066,179, taught that by applying a thin coat of gold to a steel current collector, the resistance between the cathode mix and the current collector would be markedly decreased since oxide formation would be minimized. In U.S. Pat. No. 3,485,675, Ruben suggested a surface carburized layer on the steel. Again, the solution taught by Ruben decreased the amount of oxide formed at the cathode-current collector interface. Moreover, both solutions taught by Ruben supplied the surface of the current collector with a continuous layer of a material which approximated the conductivity of bare metal. However, with the price of gold at least ten times greater today than when Ruben proposed its use as a coating for the current collector and the well-known expense and difficulty of obtaining a carburized surface layer on steel, other ways of reducing contact resistance in alkaline electrochemical cells are needed.
To avoid the expensive solutions for reducing the contact resistance between the cathode mix and the positive current collector proposed by Ruben, in Japanese Patent Publication No. 42-25145, Uchida et al. proposed coating the entire inner surface of the positive current collector with a graphite laden synthetic resin. However, since it is well known that placing any material which is less electrically conductive than steel between the cathode mix and the positive current collector increases the electrical resistance of the current collector (and graphite is at least one order of magnitude less conductive than steel and most synthetic resins are orders of magnitude less conductive than steel), the '145 reference proposes using substantial amounts of graphite in the resin. In a similar teaching, Shinoda et al. disclose in Japanese Patent Publication No. 48361-1983 a resinous coating having a high amount of carbonaceous material, wherein the coating contains between 50 and 70 weight percent carbon, and the coating is disposed over the entire inner surface of the positive current collector.
The amounts of carbon proposed for use in the synthetic resins by the Japanese references are so high that it is very unlikely that the resinous coatings disclosed therein would adhere to a positive current collector well enough for further processing. Moreover, even if the coatings did adhere to a metal current collector, they would probably lack sufficient mechanical integrity to remain attached thereto. For example, a small mechanical shock, such as placing a battery in a device, might cause the coating to become dislodged from the metal current collector. Moreover, both references disclose coatings disposed continuously over the entire inner surface of the positive current collector, thereby making the manufacture of cells according to those inventions a slow and expensive process. Consequently, the problems inherent with such large amounts of carbon in a dry coating, together with the requirement that the coating be continuous, cause the search for a more complete solution to the problem to continue.
While all of the aforementioned solutions have been attempted, the present commercial solution involves plating a steel current collector with nickel. While less expensive than plating a surface with gold or providing such a surface with a carburized layer, nickel plating has been found to be an expensive solution to the problem of decreasing contact resistance, without increasing internal cell resistance. Nickel plating of steel has been though to be of such importance that some battery manufacturers have used resinous coatings such as were disclosed in the aforementioned Japanese references on nickel plated steel current collectors. (It is interesting to note that the aforementioned Japanese references do not propose using the various resins on unplated steel current collectors.) Consequently, the ability to use a steel current collector without providing for an expensive to apply continuous surface layer has long been sought by the manufacturers of alkaline electrochemical cells.